Dark Emission when using MixRGB with Colour and Image
You should plug the alpha output of your image texture into the alpha input of the principled BSDF, then use the color output directly in the color input of the BSDF.

Make sure to check the blend mode to something else than "opaque" in the settings of the material. Press N to open the panel when in the shading node editor then go to Options>Settings>Blend Mode. Depending on your blender version you might have to try different ones to achieve your goal.